Weight reduction of an internal combustion engine, particularly that of its movable valve system, contributes vastly to improved performance of the engine. Among other parts of the movable valve system, the rocker-arm is required not only to be light-weight but also to have a small valve-equivalent weight (good inertia efficiency) to minimize pad surface wear and pitting. However, the requirements of the rocker-arm include high strength and rigidity of its body, excellent wear resistance and anti-pitting characteristics of its pad surface, and ease of valve clearance adjustment.
Thus, light weight is required. On the other hand, in view of its requirements for strength, wear resistance, anti-pitting characteristics of its pad surface, and ease of valve clearance adjustment, the rocker-arm has been mainly fabricated of a high strength cast iron or a specially treated steel. Some rocker-arms fabricated of aluminum alloy and which can be mentioned, are the rocker-arms for GM's "Electra" automobile with overhead valve (OHV) engine and for BMW's vehicle with overhead cam (OHC) engine.
In the case of GM's "Electra" the engine is an OHV using a hydraulic oil tappet and operated at large capacity and low rpm. Meanwhile, BMW's rocker-arm uses a steel eccentric ring at the valve clearance adjusting part, which is tightened with a steel bolt and nut. In such a construction, adjusting the valve clearance while the engine is running is difficult. In addition, adoption of the steel bolt and nut reduces the advantage of using an aluminum alloy rocker-arm body for the purpose of weight reduction.
In an effort to solve the problem, the present inventors proposed a rocker-arm of conventional form which is fabricated of an ordinary aluminum alloy; the anti-wear pad at one end is encasted; and the female thread to receive the adjusting screw at the other end is strengthened with a helical spring type thread insert. As compared with the conventional iron base rocker-arm, the proposed rocker-arm permitted considerable reduction of weight, but left much room to be improved, in performance and cost, etc., because of the necessity for a separate bearing bushing between the rocker-arm and the rocker-shaft.